linux-kernel - Re: [PATCH] drm/sun4i: link in front-end code if needed

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Date:   Mon, 26 Feb 2018 10:31:51 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Maxime Ripard <maxime.ripard@...tlin.com>
Cc:     Chen-Yu Tsai <wens@...e.org>, David Airlie <airlied@...ux.ie>,
        Jernej Skrabec <jernej.skrabec@...l.net>,
        Icenowy Zheng <icenowy@...c.io>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jonathan Liu <net147@...il.com>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm/sun4i: link in front-end code if needed

On Mon, Feb 26, 2018 at 10:21 AM, Maxime Ripard
<maxime.ripard@...tlin.com> wrote:
> Hi,
>
> On Fri, Feb 23, 2018 at 02:06:52PM +0100, Arnd Bergmann wrote:
>> When the base sun4i DRM driver is built-in but the back-end is
>> a loadable module, we run into a link error:
>>
>> drivers/gpu/drm/sun4i/sun4i_drv.o: In function `sun4i_drv_probe':
>> sun4i_drv.c:(.text+0x60c): undefined reference to `sun4i_frontend_of_table'
>>
>> The dependency is a bit tricky, the best workaround I have come up
>> with is to use a Makefile hack to to interpret both
>> CONFIG_DRM_SUN4I_BACKEND=m and CONFIG_DRM_SUN4I_BACKEND=y
>> as a directive to build the front-end the same way as the main module.
>>
>> Fixes: dd0421f47505 ("drm/sun4i: Add a driver for the display frontend")
>> Signed-off-by: Arnd Bergmann <arnd@...db.de>
>> ---
>>  drivers/gpu/drm/sun4i/Makefile | 5 ++++-
>>  1 file changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/gpu/drm/sun4i/Makefile b/drivers/gpu/drm/sun4i/Makefile
>> index 582607c0c488..db14e023256b 100644
>> --- a/drivers/gpu/drm/sun4i/Makefile
>> +++ b/drivers/gpu/drm/sun4i/Makefile
>> @@ -25,6 +25,9 @@ obj-$(CONFIG_DRM_SUN4I)             += sun4i-tcon.o
>>  obj-$(CONFIG_DRM_SUN4I)              += sun4i_tv.o
>>  obj-$(CONFIG_DRM_SUN4I)              += sun6i_drc.o
>>
>> -obj-$(CONFIG_DRM_SUN4I_BACKEND)      += sun4i-backend.o sun4i-frontend.o
>> +obj-$(CONFIG_DRM_SUN4I_BACKEND)      += sun4i-backend.o
>> +ifdef CONFIG_DRM_SUN4I_BACKEND
>> +obj-$(CONFIG_DRM_SUN4I)              += sun4i-frontend.o
>> +endif
>
> Can we just create a Kconfig symbol, selected by DRM_SUN4I_BACKEND,
> and we would export the symbol?

Yes, I think that would work just as well. Something like

config DRM_SUN4I_FRONTEND
      tristate
      default DRM_SUN4I if CONFIG_DRM_SUN4I_BACKEND != n

should do the trick.

      Arnd